Yes he will need to dry the core. The styrene will need to dry out completely. It could rot out the foam if left

thanks for that, i'll pass it on, looks like he is going to be riding his aircraft carrier for a while.

Sorry to hear,
AA gives good advice, Its is a long shot and you may only be prolonging the agony of water rotting the core.
Its been my experience ,because the styrene is of granular construction made up of small cells water will wick and travel around the cells of styrene and is very hard to displace completely ,even with heat.
My humble advice would be - firstly get some professional advice, one good source would be , Kracka surf in Newcastle and see what they suggest, they have a good reputation for repairing a lot of surf club epoxy boards .(I have no connection with them apart from hearing good things especially with repairs,look them up on the net).
Secondly ,This is what I think,
This is drastic but my method would be to cut around the vent ,say 100 x 100mm sq to start , (may have to enlarge depending on affected area) remove and inspect core, extract and replace affected styrene ,replace styrene with higher density styrene than parent core density, sick down and sand to suit, replace the vent with a one way vent (set and forget) ,re epoxy and put it down to experience.
Its the only sure method of knowing that the board is 100% water tight and will last.
OR
Buy your mate some tissues watch Titanic and start saving for a new board.
haha only joking, best of luck

Please write back and let us know how you go ,for future reference.
cheers
Red

ouch, cheers guys, I don't think he is a Titanic kinda guy, I am glad that I got rid of the board that I had with the vent in it. Would have not liked to be in the same situation.

Will pass on everyones advice and post back with what he does, cheers all